Redox polymer energy storage system

ABSTRACT

An energy storage system includes, in an exemplary embodiment, a first current collector having a first surface and a second surface, a first electrode including a plurality of carbon nanotubes on the second surface of the first current collector. The plurality of carbon nanotubes include a polydisulfide applied onto a surface of the plurality of nanotubes. The energy storage system also includes an ionically conductive separator having a first surface and a second surface, with first surface of the ionically conductive separator positioned on the first electrode, a second current collector having a first surface and a second surface, and a second electrode including a plurality of carbon nanotubes positioned between the first surface of the second current collector and the second surface of the ionically conductive separator.

BACKGROUND OF THE INVENTION

The field or the invention relates generally to a redox polymer energystorage system, and more particularly to the use of electroactivepolymers applied to carbon structures.

Capacitors having a double layer capacitance may be used as energystorage devices that store and release energy. Known double layercapacitors store an amount of energy that is inversely proportional tothe thickness of the double layer. At low voltages, these double layercapacitors typically have a higher energy density than conventionaldielectric capacitors.

Lithium ion batteries are also used as energy storage devices. Suchbatteries typically have very high energy density. Drawbacks of lithiumion batteries are that these batteries need to be hermetically sealedand requires a water free electrolyte composition. In addition, when thelithium ion batteries are spent, the lithium in the battery needs to besequestered rather than be placed in a waste dump.

It would be desirable to provide an energy storage system that is asolid state device, has high energy density, is not susceptible toleaking, and is stable at high G-forces and temperature extremes.

BRIEF DESCRIPTION OF THE INVENTION

In one aspect, an energy storage system is provided. The energy storagesystem includes a first current collector having a first surface and asecond surface, a first electrode including a plurality of carbonnanotubes on the second surface of the first current collector. Theplurality of carbon nanotubes include a polydisulfide applied onto asurface of the plurality of nanotubes. The energy storage system alsoincludes an ionically conductive separator having a first surface and asecond surface, with first surface of the ionically conductive separatorpositioned on the first electrode, a second current collector having afirst surface and a second surface, and a second electrode including aplurality of carbon nanotubes positioned between the first surface ofthe second current collector and the second surface of the ionicallyconductive separator.

In another aspect, an energy storage system is provided. The energystorage system includes a first current collector and a second currentcollector having a plurality of carbon nanotubes bonded to a surface ofthe first current collector and the second current collector, apolydisulfide deposited onto the carbon nanotubes of at least onecurrent collector, and an ionically conductive separator sandwichedbetween the carbon nanotubes of the first current collector and thenanotubes of the second current collector. The separator beingconfigured to transport protons from the carbon nanotubes of the firstcurrent collector to the carbon nanotubes of the second currentcollector.

In another aspect, a method of making an energy storage system isprovided. The method includes applying a plurality of carbon nanotubesto a surface of a first current collector, applying a plurality ofcarbon nanotubes to a surface of a second current collector, and coatingthe carbon nanotubes of at least one of the first current collector andthe second current collector with a polydisulfide. The method alsoincludes positioning an ionically conductive separator between thecarbon nanotubes applied to the first current collector and thenanotubes applied to the second current collector to form the energystorage system having a sandwich configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an embodiment of a redox polymerenergy storage system.

FIG. 2 is a schematic illustration of another embodiment of the redoxpolymer energy storage system.

FIG. 3 is a schematic illustration of a super-capacitor of a firstconfiguration.

FIG. 4 is a schematic illustration of a super-capacitor of a secondconfiguration.

FIG. 5 is a schematic illustration of a super-capacitor of a thirdconfiguration.

FIG. 6 is a graph of cyclic voltammetry of the super-capacitors shown inFIGS. 3-5.

FIG. 7 is a graph of chronocoulometry of the super-capacitors shown inFIGS. 3-5.

DETAILED DESCRIPTION OF THE INVENTION

A redox polymer energy storage system and method of making the energystorage system is described in detail below. The energy storage systemuses electro-active redox polymers for energy storage, for example, inone embodiment, polydisulfides that are coated and/or otherwise appliedto carbon nanotube (CNT) structures. In another embodiment, a disulfidemonomer is applied to the CNT structures and may be electro-polymerizedto form a polydisulfide. The resulting CNT-polydisulfide structureserves as a redox electrode for the energy storage system. Thepolymerization reaction of the disulfide monomer is reversible andpermits the energy storage system to withstand multiple charge anddischarge cycles. The energy storage system includes two currentcollectors with a first electrode formed from carbon nanotubes, a secondelectrode of carbon nanotubes, and a transport separator, positionedbetween the two current collectors. The transport separator ispositioned between the first and second electrodes to insulate theelectrodes from electrical contact. The energy storage system is a solidstate device that does not utilize liquids and thus is not susceptibleto leaking and is stable at high G-forces and temperature extremes. Inaddition, the energy storage device mechanism makes and breaks disulfidebonds, and thus only protons transport during the charge and dischargecycles. Because the transport separator only needs to facilitate protontransport, the transport separator facilitates high power density in theenergy storage system.

Referring to the drawings, FIG. 1 is a schematic illustration, in anexemplary embodiment, of a redox polymer energy storage system 10.Energy storage system 10 has a sandwich configuration of a plurality oflayers. The layers include a first current collector 12 and a secondcurrent collector 14. A first electrode 16 formed from carbon nanotubes18 (CNT) applied to an inner surface 20 of first current collector 12,and a second electrode 22 formed from carbon nanotubes 18 applied to aninner surface 23 of second current collector 14. An ionically conductiveseparator 24 is positioned between first electrode 16 and secondelectrode 22.

Carbon nanotubes 18 have an inherently large surface area and thus alarge capacitance (C) because C is proportional to the surface area A ofcarbon nanotubes 18. (C=∈∈° A/d where ∈ is the dielectric permittivityof the electrolyte double layer, ∈° the dielectric permittivity of freespace and d is the double layer thickness). To enhance the energystorage capability of carbon nanotube electrodes 16 and 22, apolydisulfide is deposited onto the surface of the carbon nanotubes. Thepolydisulfide enhances the energy storage capability of the electrode byadding a redox capacitance to the existing carbon nanotube capacitance.The magnitude of the redox capacitance is proportional to the moleculardensity of the disulfide molecule on the carbon nanotube surface.

An electroactive redox polymer, for example, polydisulfide, is appliedto carbon nanotubes 18. In another embodiment, a disulfide monomer isapplied to carbon nanotubes 18, and the disulfide monomer iselectro-polymerized to form a polydisulfide on carbon nanotubes 18. Inthe exemplary embodiment, the polydisulfide may be apoly(2,5-dimercapto-1,3,4-thiadiazole) (polyDMcT). Other suitabledisulfide polymers may include, but not limited to,bis-2,5-dithio-1,3,4-thiadizole,poly(Zn-2,5-dimercapto-1,3,4-thiadiazole),poly(Cu-2,5-dimercapto-1,3,4-thiadiazole),poly(AI-2,5-dimercapto-1,3,4-thiadiazole),poly(Fe-2,5-dimercapto-1,3,4-thiadiazole), and the like.

Ionically conductive separator 24 transports protons between firstelectrode 16 and second electrode 22 as storage system is charged anddischarged. Separator 24 includes an acid polymer gel coated on thesurfaces of separator 24. The acid polymer gel may be formed from asilicontungstic acid (SiWA). In another embodiment, the SiWA isdispersed in polyDMcT, and applied to first and second nanotubeelectrodes 16 and 22. Because only protons are transported throughseparator 24, high rates of transport are facilitated and provide forlow equivalent series resistance (ESR) high power density devices. Theenergy of storage system 10 may be calculated from the equation C=½(Cdl+Credox)V², thus the energy is based on the surface area of the CNTand the molecular density of the DMcT. Power density is proportional toV²/4ESR thus as ESR decreases, power density increases.

The disulfide, for example, 2,5-dimercapto-1,3,4-thiadiazole (DMcT), maybe electropolymerized to polyDMcT. The DMcT polymerization reaction isreversible as shown below.

The DMcT is polymerized to polyDMcT during a charging cycle and revertsback to DMcT during a discharge cycle of storage system 10. For example,at the anode electrode DMcT polyDMcT, and at the cathode electrode,polyDMcT→DMcT.

First current collector 12 and a second current collector 14 may beformed from any conductive material. Suitable conductive materials mayinclude, but are not limited to, metals, carbon, graphite, and compositematerials, for example, polymers containing carbon fibers or particles,graphite, and metal fibers or particles.

In another embodiment, as best shown in FIG. 2, a redox polymer energystorage system 40 has a sandwich configuration of a plurality of layersas similar to energy storage system 10 described above. The layersinclude a first current collector 42 and a second current collector 44.A first electrode 46 formed from carbon nanotubes 48 (CNT) applied to aninner surface 50 of first current collector 42, and a second electrode52 formed from carbon nanotubes 48 applied to an inner surface 51 ofsecond current collector 44. An ionically conductive separator 54 ispositioned between first electrode 46 and second electrode 52. Firstelectrode 46 includes a polydisulfide deposited onto the surface of thecarbon nanotubes 48. Second electrode 52 does not include apolydisulfide nor a disulfide. Separator 54 includes an acid polymer gelcoated on the surfaces of separator 54. The acid polymer gel may beformed from a silicontungstic acid (SiWA).

The DMcT is polymerized to polyDMcT during a charging cycle and revertsback to DMcT during a discharge cycle of storage system 40. For example,at the anode electrode DMcT→polyDMcT, and at the cathode electrode,H⁺→H⁺ CNT⁻.

Sample tests were performed to show the ability of super-capacitors tostore energy. Three different super-capacitor configurations weretested. Type 1 configuration 60, shown in FIG. 3, included two carboncomposite electrodes 62 and 64 having a coating 66 of a blend ofpolyDMcT and SiWA applied to inner surfaces 68 and 70 of electrodes 62and 64 respectively. An ionically conductive separator 72 was positionedbetween electrodes 62 and 64. Separator 72 was a piece of Whatman 1qualitative filter paper soaked in SiWA and air dried for about 20minutes. The Type 1 super-capacitor was assembled by stacking electrodes62 and 64 as shown in FIG. 3 with separator 72 between the electrodes.

Type 2 configuration 80, shown in FIG. 4, included two carbon compositeelectrodes 82 and 84. A coating 86 of a blend of polyDMcT and SiWA wasapplied to an inner surface 88 of electrode 82. A coating 90 of SiWA wasapplied to an inner surface 92 of electrode 84. An ionically conductiveseparator 94 was positioned between electrodes 82 and 44. Separator 94was a piece of Whatman 1 qualitative filter paper soaked in SiWA and airdried for about 20 minutes. The Type 2 super-capacitor was assembled bystacking electrodes 82 and 84 as shown in FIG. 4 with separator 94between the electrodes.

Type 3 configuration 100, shown in FIG. 5, included two carbon compositeelectrodes 102 and 104 having a coating 106 of SiWA applied to innersurfaces 108 and 110 of electrodes 102 and 104 respectively. Anionically conductive separator 112 was positioned between electrodes 102and 104. Separator 112 was a piece of Whatman 1 qualitative filter papersoaked in SiWA and air dried for about 20 minutes. The Type 3super-capacitor was assembled by stacking electrodes 102 and 104 asshown in FIG. 5 with separator 112 between the electrodes.

Cyclic voltammetry was measured for Types 1-3. A graph shows the cyclicvoltammetry in FIG. 6. The graph is an X-Y graph of potential (volts)versus current (amps). Chronocoulometry at a charge of 0 to 1.0 volt for300 seconds was also measured for Types 1-3. A graph shows theChronocoulometry in FIG. 7. The graph is an X-Y graph of charge(coulombs) versus time (seconds). As shown in FIGS. 6 and 7,configuration Types 1 and 2 showed to be superior to configuration Type3.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal language of the claims.

What is claimed is:
 1. An energy storage system comprising: a firstcurrent collector having a first surface and a second surface; a firstelectrode comprising a plurality of carbon nanotubes on said secondsurface of said first current collector; an ionically conductiveseparator having a first surface and a second surface, said firstsurface of said ionically conductive separator positioned on said firstelectrode; at least one layer of a blend comprising polydisulfide andsilicotungstic acid positioned between said first current collector andsaid ionically conductive separator, and applied onto a surface of saidplurality of carbon nanotubes; a second current collector having a firstsurface and a second surface; and a second electrode comprising aplurality of carbon nanotubes between said first surface of said secondcurrent collector and said second surface of said ionically conductiveseparator, wherein said ionically conductive separator facilitates onlyproton transport between said first and second electrodes.
 2. The energystorage system in accordance with claim 1 wherein said plurality ofcarbon nanotubes of said second electrode comprising a polydisulfideapplied onto a surface of said plurality of nanotubes.
 3. The energystorage system in accordance with claim 1 wherein said polydisulfidecomprises at least one of poly(2,5-dimercapto-1,3,4-thiadiazole),bis-2,5-dithio-1,3,4-thiadizole,poly(Zn-2,5-dimercapto-1,3,4-thiadiazole),poly(Cu-2,5-dimercapto-1,3,4-thiadiazole),poly(Al-2,5-dimercapto-1,3,4-thiadiazole), andpoly(Fe-2,5-dimercapto-1,3,4-thiadiazole).
 4. The energy storage systemin accordance with claim 1 wherein said polydisulfide iselectropolymerized from a 2,5-dimercapto-1,3,4-thiadiazole monomer. 5.The energy storage system in accordance with claim 1 wherein saidionically conductive separator comprising an acid polymer gel comprisinga silicotungstic acid.
 6. An energy storage system comprising: a firstcurrent collector and a second current collector having a plurality ofcarbon nanotubes bonded to a surface of said first current collector andsaid second current collector; at least one layer of a blend comprisingpolydisulfide and silicotungstic acid deposited onto said carbonnanotubes of at least one of said first and second current collectors;and an ionically conductive separator sandwiched between said carbonnanotubes of said first current collector and said nanotubes of saidsecond current collector such that said at least one layer is positionedbetween said ionically conductive separator and said at least one ofsaid first and second current collectors, said separator beingconfigured to transport only protons from said carbon nanotubes of saidfirst current collector to said carbon nanotubes of said second currentcollector.
 7. The energy storage system in accordance with claim 6wherein said plurality of carbon nanotubes of said first and secondcurrent collectors comprising said polydisulfide applied onto a surfaceof said plurality of nanotubes.
 8. The energy storage system inaccordance with claim 6 wherein said polydisulfide comprises at leastone of poly(2,5-dimercapto-1,3,4-thiadiazole),bis-2,5-dithio-1,3,4-thiadizole,poly(Zn-2,5-dimercapto-1,3,4-thiadiazole),poly(Cu-2,5-dimercapto-1,3,4-thiadiazole),poly(Al-2,5-dimercapto-1,3,4-thiadiazole), andpoly(Fe-2,5-dimercapto-1,3,4-thiadiazole).
 9. The energy storage systemin accordance with claim 6 wherein said polydisulfide iselectropolymerized from a 2,5-dimercapto-1,3,4-thiadiazole monomer. 10.The energy storage system in accordance with claim 6 wherein saidionically conductive separator comprising an acid polymer gel comprisinga silicotungstic acid.